Commercial processes for producing adiponitrile, an important intermediate in the manufacture of nylon-6,6 and related products, typically include a stage in which 3-pentenenitrile (3PN) or 4-pentenenitrile (4PN) is hydrocyanated in the presence of nickel (0) catalyst to form adiponitrile. It is known that cis-2-pentenenitrile (c2PN) is formed as a byproduct during such a hydrocyanation. The formation of c2PN represents an adiponitrile yield loss in the process. Furthermore, the accumulation of c2PN during the hydrocyanation reaction is undesirable because it behaves as a catalyst poison. However, the removal of c2PN is not straightforward. It can be separated from unreacted 3PN by distillation. Alternatively, it can be removed by reaction with an alkali metal sulfite and bisulfite solution but this can complicate the procedure. With this in mind, rather than physically removing the c2PN, efforts have focused on converting it to a useful product. In this regard, it may be isomerized to 3PN, which can then be recycled back into the hydrocyanation reaction.
The isomerization of c2PN to 3PN has been described in U.S. Pat. No. 3,526,654 and U.S. Patent Publication No. 2006/0194979. Both describe that the isomerization reaction may be carried out in the presence of aluminum oxide catalyst. In U.S. Pat. No. 3,526,654, the catalyst is disclosed to be weakly basic solid state material selected from the class consisting of silica gel, alumina, and sodium-calcium aluminosilicate, with Alcoa F-1 aluminum oxide exemplified; while in U.S. Patent Publication No. 2006/0194979, the aluminum oxide catalyst has a BET surface area at least 50 m2/g.
Catalysts known for the Markovnikov addition of HCN to activated olefins tend to polymerize the activated olefin and the HCN as well. Other catalysts capable of the Markovnikov addition of HCN to olefins are not effective for non-activated olefins such as 3PN and 2-methyl-3-butenenitrile (2M3BN), which do not have an activating group in the alpha-position. U.S. Pat. No. 2,904,581 discloses that addition of HCN across activated olefins can be accomplished using tetracyanonickelate (II) salts as catalyst. However, conversions are low (less than 15%) when the olefin is α, β-disubstituted. U.S. Pat. No. 4,367,179 discloses that supported Group IA, and Group HA metals are effective catalysts for the addition of RCN to activated olefins to yield the Markovnikov addition products. However, the highly basic nature of the supported alkali and alkaline earth metal catalysts necessitates that they be used in a vapor phase process to avoid the problem of polymerization of both the HCN and the activated olefin.
U.S. Pat. No. 7,371,884 discloses that certain amines are useful as homogeneous catalysts for the Markovnikov addition of HCN to acrylonitrile to produce succinonitrile. However, a separation process for separating the catalyst from the reaction product is required after the reaction.
U.S. Pat. No. 8,394,981 discloses that certain homogenous biphosphite nickel complexes are useful as catalysts for the conversion of c2PN to greater than 90% adiponitrile, with minor formation of ethylsuccinonitrile (ESN).
None of the above publications teach a process for addition of HCN across olefins in the liquid or vapor phase, and, in particular, to the use of the specific aluminum oxide required for the present process to catalyze such a reaction. A simple, economical, improved process for addition of HCN across olefins in liquid or vapor phase hydrocyanation is provided by the present invention.